Fluorescent lamps are gas discharge lamps that are based on Hg vapor which, when excited, provides a low intensity spectral line of visible light and several high intensity lines of ultra-violet light, that are converted to visible light by the phosphor coating on the interior surface of the lamps. Fluorescent lamps were perfected as an alternative to incandescent lamps, and have since replaced the incandescent lamps in most commercial and industrial applications. The fluorescent lamp has a substantially longer life than the incandescent lamp which results in reduced maintenance costs. The fluorescent lamp also provides a more distributive light source which is two to six times more efficient than incandescent lighting in terms of luminous flux per unit of electric power consumed.
Since the fluorescent lamp has no inherent current limiting mechanism when operated by a voltage source, the fluorescent lamp requires an auxiliary device to first ignite the lamp arc and then, after ignition has occurred, to control the amplitude of the arc current. Without an auxiliary device to stabilize or limit the arc current, the lamp arc would exceed its current rating and thus, the fluorescent lamp would be damaged. In conventional systems the auxiliary device has been combined into a single device called a ballast. The ballast provides a means for igniting the lamp arc and also provides a fixed value of arc current to the lamps. A shortcoming of the fixed value of arc current lighting is that it wastes energy. Underlighted conditions are often due to light absorbing dust on the lamp and the deterioration of the phosphor coating on the inside wall of the fluorescent tube. To reduce the effect of the underlighted conditions, designers overlight the area when the lamps are new and lamina are clean so there is still sufficient light remaining when lamp light output reaches depreciated states. Therefore, much of the electric energy that can be saved by using fluorescent lighting is lost due to the industrial practices of maintaining the use of fixed value arc current lamp operation.
One prior art technique used to reduce wasteful overlighting and promote energy savings is disclosed in U.S. Pat. No. 5,483,127 to Widmayer et al. The Widmayer et al. patent discloses a fluorescent lighting control system which automatically adjusts the arc current to a fluorescent gas discharge lamp. The variable arc current lighting system includes a sensor that senses ambient light and the output light of the lamp and provides a corresponding electrical signal to an electronic circuit. The electronic circuit controls the frequency of repetition of alternating on-off periods of electronic switches. As the frequency of switching the electronic switches is increased or decreased, the effective impedance value of the current limiting inductances that are connected in series with each lamp is controlled. Thus, the current amplitude is increased or decreased by controlling the switching frequency of the electronic switches. By reducing the arc current supplied to a fluorescent lamp, the lamp operates at less than rated wattage thereby reducing electrical consumption. The variable-arc lighting system also includes a start-up circuit which provides a voltage supply to the internal electronic circuits. However, this lighting control system is complex, expensive to produce and difficult to troubleshoot and repair.
Another prior art variable-arc lighting system is the Mark VII system made by Precision Lighting, Inc., of Rockville, Md. The Mark VII system operates on the same principle as the Widmayer et al. patent but has been simplified to reduce cost and size. The Mark VII system includes electronic circuits that control the switching frequency of electronic switches in order to control the arc current in a fluorescent lamp. The Mark V11 system also includes a start-up circuit which provides a voltage supply to various internal electronic circuits.
One disadvantage of the start-up circuits in the Widmayer et al. patent and in the Mark VII system is that the start-up circuits are generally unreliable. The start-up circuit includes a power transistor that is driven on and off to provide a voltage supply to the internal electronic circuits. When the start-up circuit has completed its operation, and the ballast is in normal operation, there is a continuous high voltage present on the power transistor. The high voltage exceeds the rating of the power transistor and over a period of time the power transistor can be damaged. Replacing the power transistor with a different type of transistor having higher voltage ratings would require a different control circuit, thus increasing the need for circuit components and, as a result, increasing costs.
Another disadvantage of the start-up circuits in the Widmayer et al. patent and in the Mark VII system is that the power transistor is not always capable of being turned off when the main input voltage source is abnormally low. If the power transistor remains on or in a conducting state for a considerable time period, the electronic elements and circuits which are electrically connected to the power transistors will receive continuous current. These electronic elements and circuits, and the transistor itself, can be damaged as a result of overheating due to the continuous current flow.
A further disadvantage of the start-up circuits in the Widmayer et al patent and in the Mark 091 system is that the start-up circuit includes a single rectifier bridge in order to provide a bias voltage to multiple electronic circuits and as a consequence, the multiple electronic circuits are not electrically isolated from each other, so that the unequal voltage requirements of the different circuits is not easily provided for.